Infrared light has longer wavelengths than visible light, and is therefore less easily scattered than visible light. Hence, infrared light has the advantage of being highly transmitted through smoke and fog. Furthermore, since heat sources such as living things and the like emit infrared light, they can be imaged even at night by using infrared light. At this time, since infrared light is not seen by the eye, even if an object is lit by infrared light at night, photographing can be performed without being noticed by the object. Furthermore, if the infrared light emitted from substances and living things serving as heat sources is detected and photographed, the temperature information of the objects can be obtained. Because of such characteristics, imaging utilizing infrared light is widely used from defense areas to surveillance cameras, fire detection cameras, and the like.
These days, “uncooled infrared solid state imaging devices” that do not need a cooling mechanism are actively developed. A uncooled, that is, heat-based infrared solid state imaging device converts incident infrared light with wavelengths around 10 μm into heat using an absorption structure, then uses some kind of thermoelectric conversion means to convert the temperature change of a thermo-sensitive unit caused by this weak heat into an electric signal, and reads out the electric signal to obtain infrared image information.
For example, there is a solid state imaging device using a p-n junction of silicon in which a constant forward current is given to convert a temperature change into a voltage change. The solid state imaging device has the advantage that an SOI (silicon on insulator) substrate can be used as a semiconductor substrate to allow mass production using manufacturing processes for LSIs (large scale integrated circuits) of silicon. Furthermore, even in the case where a large number of infrared detection pixels are arranged in a matrix configuration and connected to a common interconnection, the rectifying properties of the p-n junction that is a thermoelectric conversion means can be utilized to obtain the function of row selection. Therefore, the solid state imaging device has also the advantage that pixels can be configured to be very simple.
One indicator of the performance of the solid state imaging device is the noise equivalent temperature difference (NETD, hereinafter referred to as “NETD”), which expresses the temperature resolution of the solid state imaging device. Reducing the NETD, that is, reducing the temperature difference of the solid state imaging device corresponding to noise is important in improving the sensitivity of the solid state imaging device.